Investigate auto generating parts of docs

Currently the CLI usage is out of sync with the latest release i.e. experimental is there, bootstrap isn't and so on.

Got me thinking this should be able to be codegened, putting this in an issue for future discussion.

Did a quick PoC just parsing the help command output
```
Usage: Cake.exe [script] [--verbosity=value]
[--showdescription] [--dryrun] [..]

Example: Cake.exe
Example: Cake.exe build.cake --verbosity=quiet
Example: Cake.exe build.cake --showdescription

Options:
--verbosity=value Specifies the amount of information to be displayed.
(Quiet, Minimal, Normal, Verbose, Diagnostic)
--debug Performs a debug.
--showdescription Shows description about tasks.
--dryrun Performs a dry run.
--exclusive Execute a single task without any dependencies.
--bootstrap Download/install modules defined by #module directives
--version Displays version information.
--help Displays usage information.
```
result is

Code for that is avail [here](https://github.com/cake-build/website/compare/develop...devlead:feature/codegencliusage), it's just a rough 5 minute PoC.

Ideally we should have better meta data i.e. if cake could report as xml, json etc. but wanted to test something end to end, basic idea is

* Have a main manually created document
* Where parts of the document is automatically generated

In the poc this is achieved using the WYAM include module and `^` preprocessor example:
```
`^"../../../generatedinput/cake-usage.md"
```